Murine monoclonal antibodies which neutralize maitotoxin will be selected using a functional assay that exploits the hemolytic action of maitotoxin. The resulting antibodies should be useful for several purposes. They will provide a means to measure the toxin content in crude extracts of fish tissues or dinoflagellate cultures by hemolysis neutralization assay. More importantly, neutralizing monoclonal antibodies may provide a means to passively protect or reverse maitotoxin poisoning. Such reagents could be humanized for clinical therapeutic applications. The antibodies may also be useful for other purposes including, studying the receptor pharmacology of maitotoxin and calcium ion channels; identifying and isolating maitotoxin-producing dinoflagellates; and, isolating purified maitotoxin by immunoaffinity chromatography. Anti-idiotypic antibodies of those which neutralize maitotoxin should also be of interest for a variety of analytical and biomedical applications.